This document provides details on using git to develop Perl.
If you are just interested in working on a quick patch,
see perlhack first.
This document is intended for people who are regular contributors to Perl,
including those with write access to the git repository.

Once you have changed into the repository directory, you can inspect it. After a clone the repository will contain a single local branch, which will be the current branch as well, as indicated by the asterisk.

% git branch
* blead

Using the -a switch to branch will also show the remote tracking branches in the repository:

% git branch -a
* blead
origin/HEAD
origin/blead
...

The branches that begin with "origin" correspond to the "git remote" that you cloned from (which is named "origin"). Each branch on the remote will be exactly tracked by these branches. You should NEVER do work on these remote tracking branches. You only ever do work in a local branch. Local branches can be configured to automerge (on pull) from a designated remote tracking branch. This is the case with the default branch blead which will be configured to merge from the remote tracking branch origin/blead.

You can see recent commits:

% git log

And pull new changes from the repository, and update your local repository (must be clean first)

% git pull

Assuming we are on the branch blead immediately after a pull, this command would be more or less equivalent to:

% git fetch
% git merge origin/blead

In fact if you want to update your local repository without touching your working directory you do:

% git fetch

And if you want to update your remote-tracking branches for all defined remotes simultaneously you can do

% git remote update

Neither of these last two commands will update your working directory, however both will update the remote-tracking branches in your repository.

This command will produce as output a description of the current state of the repository, including modified files and unignored untracked files, and in addition it will show things like what files have been staged for the next commit, and usually some useful information about how to change things. For instance the following:

This shows that there were changes to this document staged for commit, and that there were further changes in the working directory not yet staged. It also shows that there was an untracked file in the working directory, and as you can see shows how to change all of this. It also shows that there is one commit on the working branch blead which has not been pushed to the origin remote yet. NOTE: that this output is also what you see as a template if you do not provide a message to git commit.

First, please read perlhack for details on hacking the Perl core. That document covers many details on how to create a good patch.

If you already have a Perl repository, you should ensure that you're on the blead branch, and your repository is up to date:

% git checkout blead
% git pull

It's preferable to patch against the latest blead version, since this is where new development occurs for all changes other than critical bug fixes. Critical bug fix patches should be made against the relevant maint branches, or should be submitted with a note indicating all the branches where the fix should be applied.

Now that we have everything up to date, we need to create a temporary new branch for these changes and switch into it:

% git checkout -b orange

which is the short form of

% git branch orange
% git checkout orange

Creating a topic branch makes it easier for the maintainers to rebase or merge back into the master blead for a more linear history. If you don't work on a topic branch the maintainer has to manually cherry pick your changes onto blead before they can be applied.

That'll get you scolded on perl5-porters, so don't do that. Be Awesome.

Then make your changes. For example, if Leon Brocard changes his name to Orange Brocard, we should change his name in the AUTHORS file:

The -a option is used to include all files that git tracks that you have changed. If at this time, you only want to commit some of the files you have worked on, you can omit the -a and use the command git add FILE ... before doing the commit. git add --interactive allows you to even just commit portions of files instead of all the changes in them.

The -m option is used to specify the commit message. If you omit it, git will open a text editor for you to compose the message interactively. This is useful when the changes are more complex than the sample given here, and, depending on the editor, to know that the first line of the commit message doesn't exceed the 50 character legal maximum.

You should now send an email to perlbug@perl.org with a description of your changes, and include this patch file as an attachment. In addition to being tracked by RT, mail to perlbug will automatically be forwarded to perl5-porters (with manual moderation, so please be patient). You should only send patches to perl5-porters@perl.org directly if the patch is not ready to be applied, but intended for discussion.

Please do not use git-send-email(1) to send your patch. See Sending patch emails for more information.

Assuming that you'd like to commit all the changes you've made as a single atomic unit, run this command:

% git commit -a

(That -a tells git to add every file you've changed to this commit. New files aren't automatically added to your commit when you use commit -a If you want to add files or to commit some, but not all of your changes, have a look at the documentation for git add.)

Git will start up your favorite text editor, so that you can craft a commit message for your change. See "Commit message" in perlhack for more information about what makes a good commit message.

After you've generated your patch you should sent it to perlbug@perl.org (as discussed in the previous section) with a normal mail client as an attachment, along with a description of the patch.

You must not use git-send-email(1) to send patches generated with git-format-patch(1). The RT ticketing system living behind perlbug@perl.org does not respect the inline contents of E-Mails, sending an inline patch to RT guarantees that your patch will be destroyed.

Someone may download your patch from RT, which will result in the subject (the first line of the commit message) being omitted. See RT #74192 and commit a4583001 for an example. Alternatively someone may apply your patch from RT after it arrived in their mailbox, by which time RT will have modified the inline content of the message. See RT #74532 and commit f9bcfeac for a bad example of this failure mode.

Be aware that many files in the distribution are derivative--avoid patching them, because git won't see the changes to them, and the build process will overwrite them. Patch the originals instead. Most utilities (like perldoc) are in this category, i.e. patch utils/perldoc.PL rather than utils/perldoc. Similarly, don't create patches for files under $src_root/ext from their copies found in $install_root/lib. If you are unsure about the proper location of a file that may have gotten copied while building the source distribution, consult the MANIFEST.

git provides a built-in way to determine which commit should be blamed for introducing a given bug. git bisect performs a binary search of history to locate the first failing commit. It is fast, powerful and flexible, but requires some setup and to automate the process an auxiliary shell script is needed.

The core provides a wrapper program, Porting/bisect.pl, which attempts to simplify as much as possible, making bisecting as simple as running a Perl one-liner. For example, if you want to know when this became an error:

perl -e 'my $a := 2'

you simply run this:

.../Porting/bisect.pl -e 'my $a := 2;'

Using bisect.pl, with one command (and no other files) it's easy to find out

Which commit caused this example code to break?

Which commit caused this example code to start working?

Which commit added the first file to match this regex?

Which commit removed the last file to match this regex?

usually without needing to know which versions of perl to use as start and end revisions, as bisect.pl automatically searches to find the earliest stable version for which the test case passes. Run Porting/bisect.pl --help for the full documentation, including how to set the Configure and build time options.

If you require more flexibility than Porting/bisect.pl has to offer, you'll need to run git bisect yourself. It's most useful to use git bisect run to automate the building and testing of perl revisions. For this you'll need a shell script for git to call to test a particular revision. An example script is Porting/bisect-example.sh, which you should copy outside of the repository, as the bisect process will reset the state to a clean checkout as it runs. The instructions below assume that you copied it as ~/run and then edited it as appropriate.

You first enter in bisect mode with:

% git bisect start

For example, if the bug is present on HEAD but wasn't in 5.10.0, git will learn about this when you enter:

You can peek into the bisecting process with git bisect log and git bisect visualize. git bisect reset will get you out of bisect mode.

Please note that the first good state must be an ancestor of the first bad state. If you want to search for the commit that solved some bug, you have to negate your test case (i.e. exit with 1 if OK and 0 if not) and still mark the lower bound as good and the upper as bad. The "first bad commit" has then to be understood as the "first commit where the bug is solved".

git help bisect has much more information on how you can tweak your binary searches.

If you want to make changes to someone else's topic branch, you should check with its creator before making any change to it.

You might sometimes find that the original author has edited the branch's history. There are lots of good reasons for this. Sometimes, an author might simply be rebasing the branch onto a newer source point. Sometimes, an author might have found an error in an early commit which they wanted to fix before merging the branch to blead.

Currently the master repository is configured to forbid non-fast-forward merges. This means that the branches within can not be rebased and pushed as a single step.

The only way you will ever be allowed to rebase or modify the history of a pushed branch is to delete it and push it as a new branch under the same name. Please think carefully about doing this. It may be better to sequentially rename your branches so that it is easier for others working with you to cherry-pick their local changes onto the new version. (XXX: needs explanation).

If you want to rebase a personal topic branch, you will have to delete your existing topic branch and push as a new version of it. You can do this via the following formula (see the explanation about refspec's in the git push documentation for details) after you have rebased your branch:

NOTE: it is forbidden at the repository level to delete any of the "primary" branches. That is any branch matching m!^(blead|maint|perl)!. Any attempt to do so will result in git producing an error like this:

As a matter of policy we do not edit the history of the blead and maint-* branches. If a typo (or worse) sneaks into a commit to blead or maint-*, we'll fix it in another commit. The only types of updates allowed on these branches are "fast-forwards", where all history is preserved.

Annotated tags in the canonical perl.git repository will never be deleted or modified. Think long and hard about whether you want to push a local tag to perl.git before doing so. (Pushing simple tags is not allowed.)

The perl history contains one mistake which was not caught in the conversion: a merge was recorded in the history between blead and maint-5.10 where no merge actually occurred. Due to the nature of git, this is now impossible to fix in the public repository. You can remove this mis-merge locally by adding the following line to your .git/info/grafts file:

Run the test suite. You might not think that one typo fix would break a test file. You'd be wrong. Here's an example of where not running the suite caused problems. A patch was submitted that added a couple of tests to an existing .t. It couldn't possibly affect anything else, so no need to test beyond the single affected .t, right? But, the submitter's email address had changed since the last of their submissions, and this caused other tests to fail. Running the test target given in the next item would have caught this problem.

If you don't run the full test suite, at least make test_porting. This will run basic sanity checks. To see which sanity checks, have a look in t/porting.

If you make any changes that affect miniperl or core routines that have different code paths for miniperl, be sure to run make minitest. This will catch problems that even the full test suite will not catch because it runs a subset of tests under miniperl rather than perl.

Simple, one-off commits pushed to the 'blead' branch should be simple commits that apply cleanly. In other words, you should make sure your work is committed against the current position of blead, so that you can push back to the master repository without merging.

Sometimes, blead will move while you're building or testing your changes. When this happens, your push will be rejected with a message like this:

To ssh://perl5.git.perl.org/perl.git
! [rejected] blead -> blead (non-fast-forward)
error: failed to push some refs to 'ssh://perl5.git.perl.org/perl.git'
To prevent you from losing history, non-fast-forward updates were rejected
Merge the remote changes (e.g. 'git pull') before pushing again. See the
'Note about fast-forwards' section of 'git push --help' for details.

When this happens, you can just rebase your work against the new position of blead, like this (assuming your remote for the master repository is "p5p"):

$ git fetch p5p
$ git rebase p5p/blead

You will see your commits being re-applied, and you will then be able to push safely. More information about rebasing can be found in the documentation for the git-rebase(1) command.

For larger sets of commits that only make sense together, or that would benefit from a summary of the set's purpose, you should use a merge commit. You should perform your work on a topic branch, which you should regularly rebase against blead to ensure that your code is not broken by blead moving. When you have finished your work, please perform a final rebase and test. Linear history is something that gets lost with every commit on blead, but a final rebase makes the history linear again, making it easier for future maintainers to see what has happened. Rebase as follows (assuming your work was on the branch committer/somework):

The switches above deserve explanation. --no-ff indicates that even if all your work can be applied linearly against blead, a merge commit should still be prepared. This ensures that all your work will be shown as a side branch, with all its commits merged into the mainstream blead by the merge commit.

--no-commit means that the merge commit will be prepared but not committed. The commit is then actually performed when you run the next command, which will bring up your editor to describe the commit. Without --no-commit, the commit would be made with nearly no useful message, which would greatly diminish the value of the merge commit as a placeholder for the work's description.

When describing the merge commit, explain the purpose of the branch, and keep in mind that this description will probably be used by the eventual release engineer when reviewing the next perldelta document.

Maintenance versions should only be altered to add critical bug fixes, see perlpolicy.

To commit to a maintenance version of perl, you need to create a local tracking branch:

% git checkout --track -b maint-5.005 origin/maint-5.005

This creates a local branch named maint-5.005, which tracks the remote branch origin/maint-5.005. Then you can pull, commit, merge and push as before.

You can also cherry-pick commits from blead and another branch, by using the git cherry-pick command. It is recommended to use the -x option to git cherry-pick in order to record the SHA1 of the original commit in the new commit message.

Before pushing any change to a maint version, make sure you've satisfied the steps in "Committing to blead" above.

Sometimes a change affects code paths which you cannot test on the OSes which are directly available to you and it would be wise to have users on other OSes test the change before you commit it to blead.

Fortunately, there is a way to get your change smoke-tested on various OSes: push it to a "smoke-me" branch and wait for certain automated smoke-testers to report the results from their OSes.

The procedure for doing this is roughly as follows (using the example of of tonyc's smoke-me branch called win32stat):

First, make a local branch and switch to it:

% git checkout -b win32stat

Make some changes, build perl and test your changes, then commit them to your local branch. Then push your local branch to a remote smoke-me branch:

As described earlier, if there are many changes on your smoke-me branch then you should prepare a merge commit in which to give an overview of those changes by using the following command instead of the last command above:

% git merge win32stat --no-ff --no-commit

You should now build perl and test your (merged) changes one last time (ideally run the whole test suite, but failing that at least run the t/porting/*.t tests) before pushing your changes as usual:

% git push origin blead

Finally, you should then delete the remote smoke-me branch:

% git push origin :smoke-me/tonyc/win32stat

(which is likely to produce a warning like this, which can be ignored:

remote: fatal: ambiguous argument 'refs/heads/smoke-me/tonyc/win32stat':
unknown revision or path not in the working tree.
remote: Use '--' to separate paths from revisions

The committers have SSH access to the two servers that serve perl5.git.perl.org. One is perl5.git.perl.org itself (camel), which is the 'master' repository. The second one is users.perl5.git.perl.org (dromedary), which can be used for general testing and development. Dromedary syncs the git tree from camel every few minutes, you should not push there. Both machines also have a full CPAN mirror in /srv/CPAN, please use this. To share files with the general public, dromedary serves your ~/public_html/ as http://users.perl5.git.perl.org/~yourlogin/

These hosts have fairly strict firewalls to the outside. Outgoing, only rsync, ssh and git are allowed. For http and ftp, you can use http://webproxy:3128 as proxy. Incoming, the firewall tries to detect attacks and blocks IP addresses with suspicious activity. This sometimes (but very rarely) has false positives and you might get blocked. The quickest way to get unblocked is to notify the admins.

These two boxes are owned, hosted, and operated by booking.com. You can reach the sysadmins in #p5p on irc.perl.org or via mail to perl5-porters@perl.org.